Several publications and patent documents are cited throughout the specification in order to describe the state of the art to which this invention pertains. Each of these citations is incorporated herein by reference as though set forth in full.
The enzymes of coagulation are trypsin-like enzymes that belong to the S1 peptidase family of proteases that bear a chymotrypsin-like fold. The coagulation proteases contain catalytic domains that are highly homologous to each other and to the ancestral serine proteases of digestion. The structural homology/identity is so great (>70%) that residues in the catalytic domains of the coagulation enzymes are numbered according to the corresponding residues in chymotrypsinogen.
The coagulation enzymes circulate in blood as inactive precursors, zymogens, that require proteolytic cleavage for activation. The zymogens possess ˜10,000-fold or less proteolytic activity when compared to the serine proteases produced following activation. Initiation of coagulation at the site of vascular damage leads to a series of reactions in which a zymogen is converted to a protease through specific proteolytic cleavage and forms the enzyme for the successive reaction. This culminates in blood cell activation and the conversion of soluble fibrinogen to insoluble fibrin and hence the formation of the clot. Excess proteases are removed by reaction with circulating protease inhibitors that act as “suicide” substrates or those that recognize the active enzymes. Thus, proteolytic activation of the coagulation zymogens is a key regulatory feature of the coagulation cascade.
Although some of the coagulation zymogens are cleaved at two or more sites in their respective activation reactions, formation of the protease requires cleavage at a single site. Cleavage at this site and its structural consequences are considered in the most facile way using the homologous numbering system based on chymotrypsinogen and the extensive structural work done with trypsinogen and trypsin. The conversion of the zymogen to serine protease requires cleavage following Arg15 (typically the bond between Arg15 and Ile16) which typically removes an activation peptide and exposes a new N-terminus in the catalytic domain beginning with Ile16. One example is the conversion of factor X to factor Xa (see FIGS. 1 and 2). In trypsin and factor Xa, the new N-terminal sequence begins with Ile16-Val17-Gly18-Gly19 (SEQ ID NO: 4). For other clotting enzymes, the new N-terminal sequence is a variation on the same theme. The N-terminal sequence then folds back into the catalytic domain and inserts into the N-terminal binding cleft in a sequence-specific manner which is referred to as “molecular sexuality”. See FIG. 2. Accordingly, variants with alternate N-terminal sequences are not likely to undergo molecular sexuality in a comparable way. N-terminal insertion leads to the formation of a salt bridge between the α-NH2 group of Ile16 and Asp194 in the interior of the catalytic domain. Salt bridge formation is associated with numerous changes in catalytic domain structure including: rearrangements of the so-called activation domains, shown in FIG. 3; formation of the oxyanion hole required for catalysis and the formation of a substrate binding site. These changes lead to the maturation of the active serine protease. The key contribution of sequence-specific interactions of the new N-terminus through molecular sexuality and salt bridge formation to the maturation of the active protease are evident from the following facts: bacterial proteases that do not require cleavage for activation utilize another side-chain within the catalytic domain to salt bridge with Asp194; trypsinogen can be activated to a proteinase-like conformation without cleavage but with extremely high concentrations of an Ile-Val dipeptide that inserts into the cleft, albeit very inefficiently; the Val-Ile dipeptide and other variants are far less effective; additionally, there are two examples of bacterial proteins that activate coagulation zymogens in the absence of cleavage by subverting the activation mechanism via provision of their own N-terminus that inserts into the N-terminal binding cleft.
The structural changes outlined above provide a molecular explanation for the conversion of a precursor zymogen to an active serine protease. However, unlike trypsin which is fully active following cleavage at Arg15, many of the coagulation enzymes act very poorly on their protein substrates. Even though they generally possess fully functional active sites and can cleave small peptidyl substrates, efficient cleavage of the biological substrate often requires a cofactor protein (FIG. 2). In these cases, the cofactor proteins increase the rate of protein substrate cleavage by several thousand fold. Although the mechanism by which the cofactor proteins function remains to be resolved, they are unlikely to function by making the protease more enzyme-like and therefore more efficient. A key point is that, with one exception, the cofactors selectively bind the protease and not the corresponding zymogen. For example, factor Xa binds with high affinity to membrane-bound FVa, whereas the zymogen factor X does not bind FVa.
Depending on the state of the patient it may be desirable to develop altered coagulation cascade proteins which possess enhanced or reduced coagulation function. It is an object of the invention to provide such proteins for use as therapeutics.